Recently, the determination of a position of an automobile which is provided with an onboard vehicle navigation system takes place by utilizing information which is available from GPS (global positioning system) satellite. Such positioning technology is disclosed, for example, in Japanese Laid-Open Patent Application No. 137,009/1986.
A satellite transmits information indicating a predetermined orbit of the satellite and an exact timing when such information is delivered from the satellite. The position of the satellite can be determined on the basis of such information, and also a distance between the satellite and a point of reception can be determined from a time delay involved with the propagation of a radio wave which carries such information. Accordingly, where information from four satellites is simultaneously available, simultaneous equations can be set up from information obtained from the individual satellites and can be solved to determine the position of the point of reception in three dimensions (latitude, longitude and altitude) and any error associated with a clock associated with a receiver in an accurate manner. Where the altitude is already known, the latitude, the longitude and any error of the clock can be detected on the basis of information available from three satellites. In addition, such positioning is also enabled if the number of available satellites is equal to two or less through an auxiliary use of an acceleration sensor, for example.
However, in actuality, it is not assured that information delivered from all the satellites is always maintained to be correct. For example, in the event the actual orbit of a satellite largely deviates from a predetermined orbit, it takes a considerable length of time until orbital information delivered from that satellite is corrected, giving rise to the likelihood that orbital information containing a large magnitude of error may be received. A positioning error of a large magnitude then results if the positioning calculation is based on such uncorrected information.
Accordingly, it is apparent that a verification to see if information delivered from a satellite or satellites is correct is very important. In particular, since the number of satellites which are simultaneously available is limited, it is necessary to verify the reliability of information from all the satellites which are then available and to conduct a positioning process on the basis of information for which any error is minimized in order to enhance the positioning accuracy. Procedures which were used in the prior art in order to verify satellite information will be described below.
By way of example, assume now that there are four satellites, A, B, C and D for which the reliability of data has been verified, and that another satellite E has now become available and requires the verification of the reliability of data from this satellite E. Initially, a positioning process is conducted on the basis of information from four verified satellites A, B, C and D to yield a positioning result Pabcd. Then, a positioning process is repeated on the basis of information from four satellites A, B, C and E to produce a positioning result Pabce. Subsequently, a positioning process with information from four satellites A, B, E and D produces a positioning result Pabed, followed by a positioning process on the basis of information from four satellites A, E, C and D to produce a positioning result Paecd, which is then followed by a final positioning process conducted on the basis of information from four satellites E, B, C and D which produces a positioning result Pebcd. These five positioning results are then paired, and a difference in each pair (Pabcd-Pabce, Pabce-Pabed, Pabed-Paecd, Paecd-Pebcd, Pebcd-Pabcd, . . . ) is examined. If all of these differences are relatively small in magnitude, it follows that information from the new satellite E has a small error involved, and hence may be utilized in the positioning process. However, if one of these differences is found to be large in magnitude, it follows that information from the satellite E involves a large error, and hence cannot be utilized.
Where an exact position of the point of reception is already known, a positioning process is conducted utilizing information from various combinations of satellites to produce respective positioning results Pabce, Pabed, Paecd, Pebcd. A difference between each of such positioning result and the actual position is examined. If all of these differences are relatively small in magnitude, it may be concluded that any error involved with information from the new satellite E is small, and hence such information can be utilized. However, if one of these differences exhibits a large magnitude, it follows that information from the satellite E may involve an error of an increased magnitude, and hence information from the satellite E cannot be utilized.
When verifying the reliability of satellite data in the manner mentioned above, it is necessary to repeat the positioning calculation for each verification, and such positioning calculation must be repeated a number of times which is equal to the number of combinations of all the satellites. However, such positioning calculation is a very complex process, unavoidably requiring an increased length of time to verify the reliability of data.